Red phosphorus is used as a smoke-producing agent in a number of ammunition items. The red phosphorus burns in the presence of oxygen to form anhydrous phosphoric acid which, when combined with moisture in the air, forms a thick white cloud of smoke. A certain amount of heat input is required to start the reaction, and this is provided by a match or ignition mixture. After the reaction is initiated, exothermic heat aids in sustaining the smoke-producing process.
If loose red phosphorus powder is ignited, it will react vigorously because adequate air is available to each particle in the powdered mass. However, when the red phosphorus is pressed into a pellet, the air voids are eliminated. In the latter situation, the oxygen is only available to the outer surface of the pellet. As a result, reaction can only occur at the surface of the pellet. Although the reaction is controllable by the latter pelletized arrangement, sufficient heat is not produced to sustain significant smoke-production. In such a situation, the smoke is often thin, and of little use in the screening of a military operation.
To produce an adequate smoke-screen, the art intimately mixes an oxygen source into the red phosphorus powder prior to pressing the same into a pellet. This solution of the cited problem was found adequate for the intended purpose of the art. The oxygen sources are called oxidizers, and include such ingredients as nitrates, chlorates, oxides and chromates. The nitrates such as sodium nitrate are preferred for use with red phosphorus to produce a military screen. Sodium nitrate is a good source of oxygen, inexpensive, substantially free of contaminants, and does not render the red phosphorus overly reactive. When the composition is overly reactive, it could be explosive.
However, there is a serious drawback with the use of oxidizers in the formulation. As you increase the concentration of oxidizer in a specific formulation, the following are increased, viz: the reaction rate, the heat output, and the sensitivity to various stimuli such as friction and impact. A concentration of oxidizer in the formulation could be reached where there would be sufficient oxygen available to react with all the red phosphorus in the formulation without the need for additional air. However, at that point, there would be excessive heat output, and the sensitivity would be increased to a dangerous level.
In the art, the concentration of oxidizer in the formulation is below the above described concentration so that a controllable reaction will occur with only sufficient heat to propogate the reaction. To be more specific, the formulation utilized in the art for the M819 contains 79.5 percent red phosphorus and 14 percent sodium nitrate. These pellets produce a very large volume of smoke in a 2.5 minute time frame. While performance of this formulation is considered quite good, it falls short of the desired 5 minute screening requirement. As described, this formulation produces a very large volume of smoke. In fact, more smoke is produced by this formulation than is necessary for adequate screening purposes. If this volume of smoke could be redistributed over a 5 minute time frame, the requirement for a sustained screen for military operations would be adequately met.